1. Field of the Invention
The present invention relates to an exhaust gas purifying filter which collects particulates composed mainly of carbon contained in the exhaust gas discharged from automotive and other internal combustion engines, whereby purifying the exhaust gas. More particularly, it relates to a structure of a ceramic exhaust gas purifying filter having the honeycomb structure, and to a process for producing the same.
2. Description of the Prior Art
A conventional cylindrical exhaust gas purifying filter having the honeycomb structure is shown in FIGS. 9 and 10. It has a plurality of passages (1, 2) which are separated from one another by partition walls (5). The openings of the passages are closed alternately checkerwise at end of the filter, and the other openings of the passages which are left open are closed at the other end of the filter. This exhaust gas purifying filter works as follows when attached to the exhaust system of an internal combustion engine. The exhaust gas enters the openings of the inward passages (2) arranged at one end of the filter. Since the inward passages (2) are blocked by the blocking material (4) at their opposite ends, the exhaust gas entering the inward passages (2) does not leave them straight, but pass through the pores in the porous partition walls (5) forming the section of the passages. Particulates in the exhaust gas are collected by the pores, and the purified exhaust gas passes through the outward passages (1) adjacent to the inward passages (2) and leaves their openings at the other end of the filter.
The exhaust gas purifying filter as mentioned above has been produced by, for example, the following method.
At first, a raw material for extrusion molding is prepared by mixing cordierite powder (prepared from talc, kaolin, alumina, etc.), methylcellulose (as a binder), water (or other fluids), and an additive to form pores. The mixing may be accomplished by using a kneader or the like. The raw material thus prepared is then formed by extrusion molding into a shape having a plurality of passages separated by latticed partition walls. The shape is dried by heating. The openings of the passages at one end of the shape are blocked alternately checerkwise with the above-mentioned mixed material. The openings of the passages at the other end of the shape are also blocked in the same manner. The shape thus prepared is dried and sintered at a proper temperature for a proper period of time. In this way, the exhaust gas purifying filter having the honeycomb structure as shown in FIGS. 9 and 10 is obtained.
The additive to form pores includes those substances such as iron powder, copper powder, and nickel powder which melt to form a liquid phase at a temperature lower than the sintering temperature of the above-mentioned mixed material, or those substances such as carbon and wax which burn or volatilize upon sintering. The pore size of the partition walls is adjusted as desired by changing the particle size of those substances and kind of the additive.
The conventional exhaust gas purifying filter as mentioned above is restricted in its performance by two factors--the efficiency in collecting particulates and the pressure loss of the exhaust gas passing through the filter. In order to optimize these contradictory characteristics, the pore diameter and the pore volume in the partition walls are designed according to the conditions under which the exhaust gas purifying filter is used. In the case of exhaust gas purifying filter disclosed in, for example, Japanese Patent Laid-open No. 70814/1983, this problem is solved by forming through-holes that connect both sides of the partition wall, whereby reducing the pressure loss at a small sacrifice of collection efficiency.
In the case of the above-mentioned exhaust gas purifying filter, the collecting efficiency is dependent on the length of time of use. In other words, the collecting efficiency is low at the beginning of a new filter, but it increases with time because particulates accumulate on the surface and pores of the partition wall. Then, the collecting efficiency reaches a constant level which is determined by the pore diameter and pore volume of the partition wall. Therefore, in the case of a new or regenerated exhaust gas purifying filter, the collecting efficiency in a certain period of time after the start of use is one half to one-third of the ultimate one. This leads to a trouble that the filter emits an exhaust gas containing a large amount of particulates in that period. This trouble may be eliminated if we use a filter having as small a pore diameter as particulates to be collected. On the other hand, such a filter causes pressure loss to increase sharply with time, decreasing its life.